It is known from Italian Pat. Nos. 869,691 and 901,929 assigned to Montedison, that an excellent bright blue vat dye containing the structure of 5, 14-diazaisoviolanthrone can be obtained from 3,3'-thiobis [2-methoxy-1-azabenzanthrone] by alkaline fusion.
According to Italian Pat. No. 869,691, 3,3'-thiobis[2-methoxy-1-azabenzanthrone] was synthesized by bromination of 2-hydroxy-1-azabenzanthrone in H.sub.2 SO.sub.4, methylation of the resulting 3-bromine-2-hydroxy-1-azabenzanthrone with (CH.sub.3).sub.2 SO.sub.4, and subsequent reaction of the methylated compound with Na.sub.2 S: ##SPC1##